Co-pyrolysis method for the preparation of fluoroaromatic compounds



F. R. CALLIHAN ETAL 3,158,657 CO-PYROLYSIS METHOD FOR THE PREPA Nov. 24, 1964 RATION OF FLUOROAROMATIC COMPOUNDS Filed Sept. 25, 1962 mzqxkwzomopiomofo 5 R A $350 m 95328 WM oi zom omoad M WM E L S Km Mm lllll R PM FM JWL ATTORNEY United States Patent 3,158,657 CO-PYROLYSKS METHGD FGR THE PREPARA- TION OF FLUGRUARGMATIC CUMPOUNDS Frank R. Callihan, Huntington Station, and Charles L.

Quatela, Great Neck, N.Y., assignors to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Sept. 25, E62, Ser. No. 226,052 7 Claims. (Cl. 260-650) The present invention generally relates to the production of fluoroaromatic compounds and, more particularly, concerns a high yield method for producing such compounds by the co-pyrolysis of dichlorofluoromethane and chlorofluoromethane.

Fluoroaromatic compounds possess many chemical and physical properties which are of outstanding value in a wide variety of applications. Said properties include, for example, thermal stability at elevated temperatures of about 1,000 E, resistmce to oxidation and resistance to radiation. Additionally, fiuoroaromatic compounds are non-explosive, non-flammable, and non-corrosive. These properties make the fiuoroaromatic compounds important in the chemical, electronic, nuclear energy, petroleum, pharmaceutical and textile industries, among others.

Although some fluoroaromatic compounds and certain of the advantageous properties have been known for some time, prior methods for the preparation of said compounds have sutfered from many difficulties which have severely restricted the use of the prior methods beyond laboratory scale. The practicality of these methods has been limited by one or more of the following difficulties, amongst others: the necessity for using relatively expensive and substantially unavailable starting materials, the use of complicated and/or costly equipment, the need of complex procedures, and the poor yields of the desired fluoroaromatic compounds in terms of the quantity of starting materials involved.

One method which substantially overcomes the disadvantages and shortcomings of prior techniques for the preparation of fiuoroaro-matic compounds is described in co-pending patent application S.N. 222,544, filed on September 10, 1962 in the name of Robert A. Falk and assigned to the present assignee. According to the method of the aforementioned application, relatively inexpensive and readily available dichlorofluoromethane gas is introduced under controlled rates of flow into a pyrolysis tube maintained at a temperature within a predetermined temperature range. The gases produced at the exhaust port of the pyrolysis tube include the iiuoroaromatic compounds hexafiuorobenzene, chloropentafluorobenzene and dichlorotetrafluorobenzene.

The present invention achieves a further substantial improvement in methods for the preparation of fluoroaromatic compounds. in particular, the present invention not only retains the basic advantages of the method described in the aforementioned coending application, but also provides for a substantially two-fold increase in the yield of important fluoroaromatic compounds as com pared to the method of the co-pending application.

A principal object of the present invention is to provide a high yield method for the preparation of fluoroaromatic compounds.

Another object is to provide a method of producing fluoroaromatic compounds utilizing low cost star-ting materials readily available from commercial suppliers.

An additional object is to provide a method for the preparation of fluoroaromatic compounds yielding readily separable reaction by-products.

Another object is to provide a method for the preparation of fiuoroaromatic compounds which does not require costly or complicated equipment and is readily adapted to large scale operation.

These and other objects of the present invention as will appear from a reading of the following specification, are achieved by the provision of a method comprising the co-pyrolysis of dichlorofluoromethane (CHCl F) and chlorofiuo-romethane (CH CIF). For purposes of the present invention, the term co-pyrolysis is defined as the method of simultaneously pyrolyzing the two compounds dichloroiiuoromethane and chlorofluoromethane in the same reaction chamber. Said two compounds may be mixed either before or after injection into the reaction chamber or pyrolysis tube. The dichlorofluoromethane and chlorofluoromcthane are introduced at controlled rates of flow into a pyrolysis tube maintained at near atmospheric pressure and which is heated to a temperature within the range from about 600 C. to about 800 C. The rates of flow of dichlorofiuoro-methane as compared to chlorofluoromethane may be best expressed by molar ratios in the range from about .8 to about 1.4.

The gases produced at the exhaust port of the pyrolysis tube include hexafluorobenzene, chloropentafluorobenzene, dichlorotetrafiuorobenzene, pentafiuorobenzene, and other compounds of fluorine, chlorine, and carbon in addition to hydrogen chloride and other soluble and/or volatile compounds. The pyrolysis vapors are preferably condensed by scrubbing with water which also serves to Wash out the hydrogen chloride and other soluble products. The desired fiuoroaromatic compounds are substantially insoluble in water. A yield including approximately 5% hexafluorobenzene, 5% chloropentafiuorobenzene and 6.5% pentafluorobenzene is realized in terms of the amounts of starting materials dichlorofiuoromethane and chlorofluoromethane utilized in the process. The condensed and insoluble pyrolysis vapors (pyrolysate) may then be subjected to any of a number of conventional purification techniques including distillation, freezefiltering and vapor phase chromatography to increase the concentrations of the desired fluoroaromatic compounds.

For a more complete understanding of the present invention, reference should be had to the following specification and to the drawing which is a simplified diagram depicting the apparatus used in the performance of the method.

Referring to the figure, a source of pressurized dichlorofluoromethane gas and a source of pressurized chloroiluoromethane gas are represented by the respective numerals 1 and 2. The dichlorofluoromethane gas is supplied by reducing valve 3 and flow meter 4 to a first input of mixing valve 5. Similarly, the chlorofiuoromethane gas is supplied by reducing valve 6 and flow meter 7' to a second input of mixing valve 5. The mixed gases at the output of valve 5 are directed to a high temperature reaction chamber or pyrolysis tube 8. The pyrolysis tube is preferably made of nickel and is packed with a suitable bafile material, for example, a plurality of small diameter nickel tubes positioned longitudinally within the pyrolysis tube, for increasing the heated reaction surface encountered by the infiuent gases. Catalyst material such as tungsten may also be inserted in the reaction chamber. The pyrolysis tube is maintained at a temperature within the range from about 600 C. to about 800 C. by a conventional automatically controlled heater unit 9.

Reducing valves 3 and 6 are adjusted in accordance with the indication of the respective flow meters 4 and 7 to maintain flow of the gases at ratios within the range from about .8 to about 1.4 moles of dichlorofitmromethane to 1 mole of chlorofiuoromethane. The total flow is maintained within the range of about 4 to about 10 moles per hour. Variation of the temperature of the pyrolysis tube, the flow rate ratio, and total flow rate have some effect upon the yield of fluoroaromatic compounds. In the case of hexafiuorobenzene, for example,

it has been found that temperatures from about 700 C. to about 730 C., a flow rate molar ratio of about 1.4- and a total flow rate of about 9 moles per hour gave the highest yield (approximately 5% hexafluorobenzene) in terms of the amount of the starting materials dichlorfiuoromethane and chlorfiuoromethane utilized.

The pyrolysis vapors issuing from pyrolysis tube 8 include hexafluorobenzene (C55 chloropentafluorobenzene (C F Cl), isomeric diehlorotetrailuorobenzenes (C -Cl F pentafluorobenzene (C F H), and hydrogen chloride (HCl). The fiuoroaromatic compounds including hexafiuorobenzene, chloropentafluorobenzene, dichlorotetrafluorobenzene, pentafluorooenzene, and other fluorocarbons on the one hand and the Water soluble output gases on the other hand are separated one from the other upon passing through water scrubber 10. Scrubber 10 is packed with chips of ceramic or other inert material and receives running tap water via inlet 11 which is provided with a nozzle giving a solid conical spray. The Water in scrubber 10 is maintained at the level of and exits via outlet 13. The hydrogen chloride and other soluble gases present in the pyrolysis vapors dissolve in the water and are removed with the water exiting from outlet 13. The desired fluoroaromatie compounds including hexalluorobenzene, chloropentafiuorobenzene, dichlorotetrailuorobenzene and pentafluorobenzene vapors condense and collect (along with other pyrolysis products) at the bottom of scrubber 10 inasmuch as they are substantially insoluble in Water and are heavier than water. Said mixture of compounds may be extracted from time to time from scrubber 10 by opening petcock 14.

Alternately, one may employ the extraction method described in co-pending application SN. 141,128 for Method for the Preparation of Hexafluorobenzene, filed on September 27, 1961 in the name of Robert A. Fall; and assigned to the present assignee. Briefly, in the alternative case, the pyrolysis vapors are acted upon by a succession of traps or sealed containers coupled to the exhaust port of the pyrolysis tube. As disclosed in the aforementioned co-pending application in connection with the extraction of hexafiuorobenzene, the first trap is maintained at a temperature of about 150 C. The second trap is maintained at a temperature of about 15 C. for the condensation of most of the produced hexafluorobenzene gas. The third and final trap is maintained at a temperature of about 78 C. for the condensation of the remainder of the hexafiuorobenzene gas. The by-products remaining in the gaseous state are vented from the third trap under atmospheric pressure.

In a representative case of the production of hexailuorobenzene in accordance with the present invention, where a 2 inch diameter, 4 foot length nickel pyrolysis tube was employed, the how rates of the diehlorofiuoromethane and chloroiluoromethane gases were adjusted to be 2,000 and 1,400 cc. per minute (8.7 and 3.8 moles per hour), respectively, as measured at room temperature. concomitantly, the pyrolysis tube was heated along a 3 foot portion to a temperature of 700 C. The pyrolysis tube was packed with a number of small diameter nickel tubes to increase the contact area and heat transfer to the iniiuent gas. The pyrolysis products were collected and were found to contain 8.4% hexafluorobenzene; this corresponded to a yield of about 5% hexafiuorobenzene as compared to the amount of starting material diehloroiluoromethane and chlorofluoromethane gases utilized. On one occasion, tungsten on alumina carrier was inserted in the pyrolysis tube as a catalyst. An increased yield of about was noted. Hexailuorobenzene of above 98% purity was obtained by preparative scale vapor phase chromatography using columns packed with trieresyl phosphate on firebrick with the columns being maintained at 60 C.

Chloropentalluorobenzene and dichlorotetrafluorobenzene of greater than 95% purity were obtained by subjecting the pyrolysate resulting from the method of the present invention to preparative scale vapor phase chromatography using columns packed withe 35% Apiezon L on fireoriek with the columns being maintained at 90 C. and 120 C., respectively. In the case of pentaiiuorobenzene, the pyrolysate was distilled to obtain an C. distillation fraction. Said fraction, in turn, was subjected to preparative scale vapor phase chromatography using columns packed with 35% tricresyl phosphate on firebriek with the columns being maintained at 60 C. This procedure yielded pentafluorobenzene of about purity.

As previously mentioned, the yield of fiuoroaromatic compounds varies depending upon the reaction chamber temperature, and the flow rates and molar ratio of the dichlorofluoromethane and chlorotluoromethane gases used. The dependency of the yield of hexafiuorobenzone, for example, on the values of the aforementioned parameters is shown in the following tabulation of the results of some typical runs.

Molar Temp, O. CHClzF ClIzOlF, ratio of Percent co ,rnin (Lo/min. Cl'lClgF to Cel CH ClF Although it is preferred that a nickel pyrolysis tube be employed in the method of the present invention, reaction chambers lined with other materials, for example, platinum, ceramic, or aluminum oxide are suitable. Nickel is preferred because it is readily available, reasonably priced, mechanically strong and relatively resistant to attack by the pyrolysis vapors.

While the invention has been described in its preferred embodiments, it is understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is: claimed is:

l. The method for the preparation of fluoroaromatic compounds comprising the steps of co-pyrolyzing dichlorotlueromethane and chlorofiuoromethane at molar ratios within the range from about 0.8 to about 1.4 at a temperature within the range from about 600 C. to about 800 C. and separating the resulting fluoroaromatic compounds from the pyrolysate.

2. The method for the preparation of hexafiuorobenzene comprising the steps of co-pyrolyzing diehlorofiuoromethane and chlorofluoromethane at molar ratios within the range from about 0.8 to about 1.4 at a temperature within the range from about 600 C. to about 800 C. and separating the resulting hexailuorobcnzene from the pyrolysate.

3. The method for the preparation of chloropentafluorobenzene comprising the steps of co-pyrolyzing dichlorofluoromethane and chlorofluoromethane at molar ratios within the range from about 0.8 to about 1.4 at a temperature Within the range from about 600 C. to about 800 C. and separating the resulting .ehloropentafluorobenzene from the pyrolysate.

4. The method for the preparation of diehlorotetrafiuorobenzene comprising the steps of eo-pyrolyzing dichlorofluoromethane and chlorofluoromethane at molar ratios within the range from about 0.8 to about 1.4 at a temperature Within the range from about 600 C. to about 800 C. and separating the resulting dichlorotetrafluorobenzene from the pyrolysate.

5. The method for the preparation of pentafluorobenzene comprising the steps of co-pyrolyzing dichlorofiuoromethane and chlorofluoromethane at molar ratios within 5 the range from about 0.8 to about 1.4 at a temperature within the range from about 600 C. to about 800 C. and separating the resulting pentafluorobenzene from the pyrolysate.

6. The method for the preparation of hexafluorobenzene comprising the steps of co-pyrolyzing dichlorofluoromethane and chlorofiuoromethane at the molar ratio of about 1.4 at a temperature within the range from about 700 C. to about 730 C. and separating the resulting hexafluorobenzene from the pyrolysate.

7. The method for the preparation of fluoroaromatic compounds comprising the steps of co-pyrolyzing dichlorofluoromethane and chlorofiuoromethane at molar ratios 6 within the range from about 0.8 to about 1.4 at a temperature within the range from about 600 C. to about 800 C. and separating the resulting fluoroaromatic compounds from other pyrolysis vapors by passing said vapors through water to form a condensate.

References Cited in the file of this patent UNITED STATES PATENTS 2,927,138 Wall et a1. Mar. 1, 1960 3,016,405 Lovejoy Ian. 9, 1962 3,033,905 Wall et a1. May 8, 1962 

1. THE METHOD FOR THE PREPARATION OF FLUOROAROMATIC COMPOUNDS COMPRISING THE STEPS OF CO-PYROLYZING DICHLOROFLUOROMETHANE AND CHLOROFLUOROMETHANE AT MOLAR RATIOS WITHIN THE RANGE FROM ABOUT 0.8 TO ABOUT 1.4 AT A TEMPER- 